Method of enhancing the photoconductive properities of a semiconductor and method of producing a semiconductor with enhanced photoconductive properties

ABSTRACT

A semiconductor material with photoconductive properties and a method of the semiconductor, wherein a base material is grown and then annealed post-growth at a temperature of 475° C. or less. It has been found that be annealing at temperatures of 475° C., or less the carrier lifetime of the material and the resistivity can be optimised so as to obtain semiconductor with useful photoconductive properties.

The present invention relates to the field of semiconductors,particularly Group III-V semiconductors, with photoconductiveproperties. These semiconductors have a variety of uses, includingphotodetection, which is important for optical communications. Thepresent invention is of particular use in the field of generation anddetection of radiation in the frequency range colloquially referred toas the TeraHertz frequency range, the range being that from 25 GHz to100 THz, particularly that in the range of 50 GHz to 84 THz, moreparticularly that in the range from 90 GHz to 50 THz and especially thatin the range from 100 GHz to 20 THz.

High purity crystalline semiconductors may be grown by a variety oftechniques. These include, but are not limited to, Molecular BeamEpitaxy (MBE), Metal Organic Vapour Phase Epitaxy (MOVPE), Chemical BeamEpitaxy (CBE) and Ultra High Vacuum Chemical Vapour Deposition(UHV-CVD). MBE is considered to offer the highest purity of grownmaterial, and to provide the best control of parameters such as layerthickness, doping density and crystalline quality. MBE can be usedthroughout the III-V and other semiconductor systems, but the technologyis most advanced and developed for the specific example of GaAs. Thepresent invention is illustrated with regard to GaAs based crystalsgrown by MBE, but may be applicable to other material systems and growthtechniques.

Photoconductive semiconductors can be fabricated using low temperaturegrowth MBE, which produces single crystal material crystalographicallyaligned to the substrate material. It also introduces defects in thecrystal structure and lattice strains which allow very short lifetimesto be realised, being typically less than 1 ps. In general, the lowerthe growth temperature, the greater the defect density, however there isa lower

to the growth temperature at which excessive crystal defects and strainlead to polycrystalline growth and accordingly increasing lifetimes. Forlow temperature growth, typically a temperature of between 200° C. and300° C. is used.

For materials such as GaAs, growth at these low temperatures by MBE isbelieved to incorporate excess arsenic into the crystal. Typically, upto 2% excess arsenic can be incorporated. This low temperature grownmaterial is referred to as LT-GaAs.

Excess arsenic may also be introduced using As ion implantation on astandard GaAs crystal to introduce the excess As into the crystallattice. The material produced by this method are referred to asAs—GaAs.

Once treated to incorporate or introduce excess arsenic, LT-GaAs orAs—GaAs, have extremely short carrier lifetimes, which has been measuredat below 100 fs. This is due to the incorporation of the excess As atomsas point defects, which have a high cross section for electron capture(electrons being the majority charge carrier in these materials). Fromthe perspective of photoconductive devices, such low lifetimes equatewith very rapid device response times. However, the point defects alsogive rise to very low device resistivities.

Where such materials are to be used in photoconductive emitters andreceivers it is also desirable for the material to have a highresistivity. A low resistivity is not desirable for two reasons.Firstly, in photoconductive devices, the dark current (i.e. the residualcurrent in the absence of illumination) is increased by ease of movementof the majority carriers, and clearly large dark currents will limit thesensitivity of any detector. Secondly, for THz emitters, it is necessaryto be able to apply a very large bias across a device without drawingdamaging current flows.

One approach for achieving a higher resistivity is to anneal As enrichedGaAs after growth. The effect of the anneal is to alter theincorporation of the excess arsenic as point defects, causing itsconglomeration into arsenic precipitates distributed throughout the GaAscrystal. For LT-GaAs, annealing may be conducted within the MBE chamberimmediately after growth, or it can be performed “ex-situ” in a suitableanneal chamber. Typically, the annealing process is for a duration of10-15 minutes at temperatures in the region of 500-600° C., as it isconsidered that such temperatures (and energies) are required to movethe elemental species within the crystal lattice. Such an annealingprocess has been found to fundamentally change the mechanisms forcarrier capture, which dramatically increases the resistivity to thepoint at which it becomes usable for photoconductive devices, but withthe penalty that the lifetimes are also increased.

There is therefore a need for a semiconductor material that bettersatisfies the dual requirements of low lifetime characteristics and highresistivity.

There is also a need for a process of creating a photoconductivesemiconductor material with improved resistivity and lifetimecharacteristics.

There is additionally the need for a radiation emitter comprising aphotoconductive semiconductor material with improved resistivity andlifetime characteristics.

There is also the need for a radiation receiver comprising aphotoconductive semiconductor material with improved resistivity andlifetime characteristics.

It is an object of the present invention to address at least one problemof the prior art.

According to a first aspect, the present invention provides a method ofenhancing characteristic properties of a semiconductor, the methodcomprising annealing a base material at a temperature of 475° C. or lessto form the semiconductor. Preferably the characteristic propertiesenhanced include carrier lifetime and resistivity.

According to a second aspect, the present invention provides a method ofproducing a semiconductor material with photoconductive properties, themethod comprising annealing a base material at a temperature of 475° C.or less so as to enhance the carrier lifetime of the material and theresistivity of the base material for use as a photoconductor.

A significant advantage of the present invention is that the annealingstep is manipulated to optimise or at least enhance the materials.Therefore, it is possible to characterise base material immediatelyafter growth and to subsequently define an anneal regime which istailored to suit a particular application.

Furthermore, the base material may be cleaved prior to annealing, sothat multiple different anneal regimes may be applied to different partsof the base material, and different individual parts of the basematerial may be optimised for multiple different end uses.

This may have particular cost and efficiency advantages for wafermanufacturers, since wafers of base material may be characterisedfollowing the first growth stage, allowing subsequent applicationspecific annealing ex-situ, or with sufficiently reliable growth for anapplication specific anneal to be performed in-situ requiring nointermediate characterisation.

The above aspects of the invention have specific application to growingthe base material by LT growth or As implantation. They are additionallyapplicable to photoconductive crystalline semiconductors that areannealed post-growth. Most preferably the base material is grown usingmolecular beam epitaxy and the annealing occurs at a temperature in therange of 250° C. and 450° C.

Although the previous discussion has generally been concerned with GaAs,the present invention may be applied to any semiconductor. Preferably,the method is performed on semiconductors which comprise As, Group III-Vor Group II-IV semiconductors with photoconductive properties and mostpreferably on GaAs, InGaAs or AlGaAs.

For ternary compounds, low temperature annealing may be performed,regardless the mole fraction ‘x’ of the compound. For example, in thecase of In_(x)Ga_(1-x)As the mole fraction may be any value from 0 to 1,thus ranging from InAs to GaAs. Further, the annealing may be performedregardless of the substrate material on which the InGaAs may be grown.

Where the semiconductor is GaAs, it is preferably grown in a molecularbeam epitaxy reactor at a temperature in the range of approximately 200°C. to 300° C. In a preferred arrangement, the molecular beam epitaxy isconducted within a growth chamber and the annealing occurs outside thegrowth chamber.

Preferably the annealing is performed for fifteen minutes or less andoccurs in a temperature range of between 250° C. and 400° C. Where thesemiconductor material is InGaAs, preferably the annealing occurs in atemperature range of approximately 350° C. and 450° C.

The annealing is preferably perfomed on the material before any contactor gate metalisation is applied to the material. The annealing is alsopreferably performed for at least 30 s.

According to a third aspect, the present invention provides a method ofdetermining optimal annealing conditions for a semiconductor materialcomprising: obtaining a first set of values indicative of resistivity ofthe material for a plurality of annealing temperatures; obtaining asecond set of values indicative of carrier lifetime of the material fora plurality of annealing temperatures; and comparing the first andsecond sets of values to determine an annealing temperature or a rangeof annealing temperatures where the carrier lifetime and the resistivityof the material are optimized.

In fourth, fifth and sixth aspects, the present invention provides asemiconductor material, a photoconductive emitter comprising asemiconductor material or a photoconductive receiver comprising asemiconductor material, where the material is formed according to any ofthe first, second and/or third aspects of the present invention.

Preferably the photoconductive emitter and receiver are a terahertzemitter and receiver respectively. More preferably, the photoconductiveemitter and receiver are of the photoconducting antenna type comprisinga photoconductive substrate and a pair of spaced apart electrodesprovided on a surface of said substrate, the substrate comprisingmaterial formed in accordance with the first and/or second aspects ofthe invention.

For both photoconductive emitters and detectors it is desirable to havethe shortest possible carrier lifetime and the largest resistance. Inthe case of some materials, e.g. GaAs, both the resistance and thecarrier lifetime increase with increasing annealing temperature.Therefore, by just comparing resistivity and lifetime data it may bedifficult to choose an optimum annealing temperature without furtherguidance. To produce a good photoconductive emitter, it is desirable tomaximize the resistance. Therefore, preferably when producingphotoconductive material for an emitter, the annealing conditions whichgive the maximum resistivity while maintaining a relatively low carrierlifetime should be used. For example, for GaAs, a carrier lifetime ofless than 300 fs is desirable.

Conversely, for photoconductive detectors, it is desirable to use theshortest lifetime possible while maintaining a relatively highresistance. The value of the resistance, which may be used, depends onthe antenna design and the detection electronics. A man skilled in theart would be able to determine the minimum acceptable resistance.

For other materials, such as InGaAs, the carrier lifetime is believed todecrease and increase with increasing annealing temperature, whereas theresistance is believed to increase and then decrease with increasingtemperature. The minimum in the carrier lifetime and the maximum in theresistance are believed to roughly coincide with one another and hencethe optimum material for both a detector and an emitter may be producedusing the same annealing conditions.

The inventors have found that preferably, InGaAs should be annealed at atemperature within the range from 350° C. to 450° C. Within thisannealing range there is a minimum in the carrier lifetime and a maximumin the resistance or resistivity. For InGaAs this allows carrierlifetimes of less than 1 ps to be realised.

Therefore, in a seventh aspect, the present invention provides aphotoconductive antenna comprising a photoconducting substrate and twoelectrodes provided on the surface of said photoconducting substrate,said photoconducting substrate comprising InGaAs having a carrierlifetime of less than 1 ps.

InGaAs having such a short carrier lifetime is of use in a wide varietyof fields. Thus, in an eighth aspect, the present invention provides aphotoconductive element comprising InGaAs, said InGaAs having a carrierlifetime of at most 1 ps.

When performing an experimental optimization procedure as describedabove it is preferable to only experiment with the annealing conditionswhere an effect on the carrier lifetime or resistance is likely to beobserved. In order to determine these annealing conditions, measurementson the excess arsenic concentration may be performed as variations inthe resistance or carrier lifetime are only likely to be observed whenthe annealing conditions are sufficient to cause a decrease in theexcess arsenic concentration. The excess arsenic concentration may bemeasured by X-ray diffraction as excess arsenic may cause the crystallattice to be strained.

Measurements of the excess arsenic concentration also allow thepreferred anneal time i.e. the anneal time above which no furtherchanges are happening to the structure of the base material.

Preferably the third aspect of the present invention further comprisesdetermining an preferred annealing duration for the material. Forexample, where the material contains As, the preferred annealingduration can be determined by obtaining a third set of values indicativeof arsenic concentration of the material for a plurality of annealingdurations and for at least one annealing temperature; and comparing theat least one third set of values with the first and second sets ofvalues to determine an annealing duration and an annealing temperaturewhich together optimize the carrier lifetime and the resistivity of thematerial.

In the case of standard THz pulsed imaging systems, it is common forGaAs to be used

as the photoconductive material for the emitters and receivers, and thismaterial is photoexcited using a Ti:Sapphire laser. However, there arecost advantages that may result from the use of a pulsed laser withwavelengths in the 1 μm region, but as these wavelengths are below theband gap of GaAs, the same emitters and receivers cannot

directly be used.

In_(x)Ga_(1-x)As has a lower bandgap than GaAs, that can be tuned byvarying the composition x. Thus, by suitable choice of the composition,materials may be produced that are accessible not only to so called 1micron lasers, but also to applications that may require any longerwavelengths, which may include the 1.3 and 1.55 μm regions oftenexploited for optical communications and fibre-optics.

However, for THz applications, InGaAs, and LT-InGaAs, have not providedmaterial with sufficiently short lifetimes and/or sufficiently highresistance. The present invention enables the use of LT-InGaAs emittersand receivers with 1 micron lasers, to provide THz systems that have acost advantage over those based on GaAs and Ti:Sapphire lasers.

According to a ninth aspect, the present invention provides aninvestigative system comprising a laser configured to emit a pump beamhaving a wavelength in the range from 1.3 and 1.55 μm, an emitterconfigured to emit emitted radiation in response to irradiation by saidpump beam and a detector for detecting said emitted radiation, whereineither or both of the emitter or detector comprise InGaAs. Preferablythe pump beam has a wavelength in the range from 1040 nm to 1070 nm.

In the ninth aspect of the invention, InGaAs is used as either theemitter or detector of an investigative system. Such a system may beused for imaging or determining composition information from structures.

If both an InGaAs emitter and detector are provided, these may be useddirectly with a laser having a wavelength in the range from 1040 nm to1070 nm. Relatively cheap lasers are available which operate in thiswavelength regime.

If only one of the emitter or detector comprise InGaAs, a frequencyconversion means may be applied in the path of radiation emitted fromthe laser. An advantage of this aspect of the invention is that a cheaplaser can be utilized in an emission/detection system while still makinguse of existing mature THz emitter or detector technology created forshorter wavelengths.

The aspects of the present invention have been primarily described withreference to their use in THz radiation emission and detection. However,the above aspects of the present invention have utility in a variety offields, including high speed switching and opto-electronics, microwaveelectronics, quantum computing and integrated circuit design.

The present invention will now be described with reference to followingnon-limiting embodiments, in which:

FIG. 1 illustrates a schematic photoconductive antenna, which may beused with photoconductive material provided in accordance with anembodiment of the present invention;

FIG. 2 schematically illustrates a wafer in an MBE chamber;

FIG. 3 schematically illustrates contact annealing apparatus;

FIG. 4 graphically illustrates measurements of excess arsenicconcentration versus annealing temperature for LT-GaAs;

FIG. 5 graphically illustrates the relationship between the lifetime andannealing temperature as well as resistance against annealingtemperature for LT-GaAs;

FIG. 6 provides a comparison of a LT-GaAs receiver created using anannealing temperature of 325° C. for a period of 10 minutes with anotherreceiver created using known techniques. The graph compares the THzpower received by each receiver as a function of frequency;

FIG. 7 provides a graph of peak splitting shift obtained through x-raydiffraction measurements against anneal time for LT-GaAs;

FIG. 8 illustrates a plot of excess arsenic concentration versus annealtime for LT-GaAs.

FIG. 9 graphically illustrates the relationship between lifetime andanneal temperature as well as resistance and anneal temperature forLT-InGaAs; and

FIG. 10 is a schematic of an imaging system comprising an emitter and adetector in accordance with an embodiment of the present invention.

FIG. 1 schematically illustrates a photoconductive antenna 1 inaccordance with an embodiment of the present invention. Thephotoconductive antenna comprises a photoconductive material produced inaccordance with an embodiment of the present invention.

The antenna 1 may be configured as either an emitter or a detector.

The photoconductive antenna comprises a photoconducting substrate 3. Twoelectrodes 5, 7 are provided on a surface of said substrate 3. Theelectrodes 5, 7 are generally triangular in shape and are arranged inmirrored relation with their apexes facing. The apexes being spacedapart by photoconducting gap 9. The facing apexes are blunted orrounded.

Both generation and detection of radiation, particular Terahertzradiation, can be effected using short lifetime pulses, which areinherently broadband and rely on ultra-fast lasers, or using continuouswave (CW) radiation which relies on continuous wave laser sources and ismonochromatic.

In the former case for emission of radiation, a sub-picosecondoptical/NIR laser pulse of appropriate wavelength is directed onto thephotoconductive antenna of FIG. 1.

Upon exposure of the substrate 3 to a pulse of suitable wavelength, theconductivity of the substrate increases by a large factor, such thatcurrent flows through the material between the electrodes 5 and 7, dueto the presence of the bias electric field applied between theelectrodes. The photo-generated current transient radiates in broadbandwith frequencies up to the Terahertz range. The current will persist fora time corresponding to the “lifetime” of the photo-created chargecarriers in the material, provided the bias field is maintained.

For the latter case of CW radiation, a photoconductive antenna of thetype shown in FIG. 1 is again employed, illuminated by two CW lasers ofslightly differing frequency. When a bias is applied between the twoelectrodes 5 and 7, the non-linear I-V characteristic of the deviceleads to photo-mixing of the two CW lasers, and re-radiation at theirdifference frequency (i.e. monochromatically). This frequency can beextended up to the THz range where THz generation is required.

While the generation mechanisms of these techniques differ, they bothrely on laser excitation and suitably fabricated photoconductivedevices. By definition, these devices exhibit an increase in theirelectrical conductivity when exposed to light of a suitable wavelength.

For detection, the photoconductive antenna 1 is operated in a similarfashion to the above. The photoconductive antenna 1 is again irradiatedusing either a sub-picosecond optical/NIR laser pulse of appropriatewavelength or by two CW lasers of slightly differing frequency. Inaddition, the radiation which is to be detected is directed onto thereverse side of the antenna 1. If the detected radiation is present, acurrent flows between the electrodes which may be measured in order toindicate the presence and strength of detected radiation.

The choice of photoconductive material for such emitters and detectorsgenerally depends upon the laser excitation source. GaAs is particularlyuseful, as its band-gap matches well to Ti:Sapphire laser wavelengthsand also to laser diode wavelengths. Ti:Sapphire lasers are commonlyused in both pulsed and CW systems and laser diodes are commonly used inCW systems. Therefore the photoconductive material is chosen so that thematerial has a band-gap that is suitable for the wavelength of thelaser.

Annealing the material at high temperatures after growth relieves thecrystal strain and shrinks the lattice, precipitating out excess arsenicinto hexagonal clusters, which increases resistivity. However, the hightemperature annealing process also has the opposite effect on thecarrier lifetime, which increases. The present invention, however, hasrecognised that by controlling the temperature, and to a lesser extentthe time, at which the post growth anneal is performed will allow thematerial to be optimised for its specific application.

Conventional wisdom is, where As related Group III-V semiconductors areconcerned, excess arsenic concentrations incorporated as point defectsneed to be reconfigured within the crystal, as the point defects reducethe resistivity of the material. To date, this has been achieved byannealing at high temperatures, as it is at high temperatures thatmaximum liberation of the arsenic occurs, and which are established byhigh quality MBE growth techniques as being required for movement ofarsenic atoms to minimise the crystalline structural energy.

FIG. 2 schematically illustrates a typical growth chamber 101 used toproduce photoconducting materials. A substrate 103, upon which thephotoconducting material is to be grown is placed on wafer holder 105.The wafer holder 105 is positioned such to face cells 107, 109 and 111which contain the constituent materials of the photoconducting material.For example, if the photoconductive material is InGaAs, the cells 107,109 and 111 will comprise In, Ga and As in elemental form.

During growth, the wafer holder 103 revolves in order to ensure that thematerial is evenly grown. Once the material is grown, it is then removedfrom the growth chamber 101 and transferred to the annealing apparatusshown in FIG. 3.

For the results which will be discussed in relation to FIG. 4, a samplewas grown using the apparatus of FIG. 2. This sample, which will betermed “wafer A” consisted of 1 micron of LT GaAs (grown at nominally230° C.), grown on 500 nm of AlAs (grown at around 600° C.) by MolecularBeam Epitaxy (MBE), and all grown on a buffered GaAs substrate (waferA). The quoted LT growth temperature has been described as “nominal”, asat low temperatures, the pyrometers used for temperature calibration ofMBE growth cease to operate accurately, and so LT growth temperaturesare determined by extrapolating graphs of heater power vs. pyrometerreadings at higher temperatures.

The apparatus of FIG. 3 is an example of apparatus which may be used forcontact annealing. The apparatus comprises an annealing chamber 113. Inthe annealing chamber 113, the sample 103 is sandwiched between twowafers of semi-insulating GaAs 115 and 117. The purpose of thesandwiching arrangement is to passivate the surface of the sample withlike semiconductor material, to prevent the liberation of the excessarsenic from the crystal. Although the sides of the sample are notpassivated in this way—and thus excess arsenic could escape through thisroute—the surface area of the sides is much smaller than that of theother two surfaces.

The apparatus also comprises heating element 119, which is provided inthermal contact with at least one of the GaAs wafers 115 and 117. Theapparatus is configured to perform rapid thermal annealing by rampingthe heating element 119 up to the target temperature as rapidly aspossible, the target temperature being measured by an accuratelycalibrated thermocouple mounted very close to the sample itself. Theramping is completed within around 10 seconds. After the appropriateduration of anneal, which in this case was 10 minutes, the temperaturefalls below 200° C. within around 30 seconds. It is to be appreciatedthat this annealing procedure occurs before any metalisation such asgates or ohmic contacts are applied.

Additionally, to further inhibit liberation, the annealing is performedin an atmosphere of nitrogen gas, although other gas mixtures, such asforming gas (a hydrogen and nitrogen mix) could equally be employed. Toachieve this, the annealing chamber 13 is provided with gas inlet 21 andoutlet 23.

Other annealing schedules could in principle be adopted, all of whichattempt, in some way to passivate the surfaces of the sample. Thesetechniques may include performing the anneal in some gaseous atmospherewithout additional approaches (common gases including, but not beinglimited to, inert gases such as nitrogen or argon, forming gas, which isa mixture of hydrogen and nitrogen, or oxygen). In the specific case ofsemiconductors containing arsenic, annealing may be performed in anarsenic

pressure environment. Alternatively, the sample surface may be coatedprior to annealing, with an example of a suitable cap being siliconnitride.

FIG. 4 is a graph of excess arsenic concentration against annealtemperature for the above described GaAs sample.

To obtain the graph of FIG. 4, an X-ray diffraction technique was used.This technique is sensitive to the strain in the crystal that is causedby the incorporation of excess As atoms on the crystalline sites whichwould otherwise be occupied by Ga atoms (referred to as As_(Ga) ⁺ sites,as the defect is associated with a positive charge).

Individual X-ray diffraction curves show two distinct peaks, as can beseen in the inset graph shown to the top right of FIG. 4, which wasmeasured at an anneal temperature of 275° C. The dominant peak at anangle of 0 arcsecs corresponds to the underlying GaAs substrate uponwhich the photoconducting layer was formed, while the weaker peak ataround −53 arcsecs is that of the LT GaAs layer, and is shifted from thedominant peak by the strain. The peak splitting can be related to theconcentration of As_(Ga) ⁺ sites, and the values derived in this way areshown plotted against the anneal temperature in the main graph of FIG.4. As the excess As is removed from the Ga sites to produce theprecipitates, the As_(Ga) ⁺ concentration decreases.

Thus the graph shows that the concentration of arsenic atoms occupyinggallium sites is lowest at the higher temperatures and that saturationoccurs when all the arsenic is liberated, which is at above 500° C. Theerror bars on the final five data points of FIG. 4 (at 400, 425, 450,475 and 500° C.) become progressively larger because at thesetemperatures the two peaks of the X-ray diffraction are not clearlyresolved, and so curve fitting routines must be employed to extracttheir positions.

However, another feature of this graph, which is key to the presentinvention, is that the excess arsenic concentration starts dropping fromabout 250° C., which indicates that even at this low temperature,arsenic is moving about within the crystal. Therefore, this illustratesthat for GaAs annealing temperatures from about as low as 250° C. couldbe utilised to precipitate out excess arsenic.

FIG. 5 is a graph plotting the relationship between the annealingtemperature on the x-axis and lifetime as well as resistance on the yaxis.

The top curve is the resistance measurement for different annealingtemperatures, and the bottom curve is the lifetime measurements fordifferent annealing temperatures. The latter is determined by a methodcalled time resolved photoreflectance, that has a minimum resolution ofaround 100 fs. Hence data points in the region of 100 fs place a maximumvalue on the lifetime; the actual lifetime may be less than this valuebut this would not be discernible from the experimental error. This datawas obtained from the material described with reference to FIGS. 2 and 3above (wafer A), with an annealing duration of ten minutes. It is to beappreciated the resistance scale utilised is on a log scale.

Furthermore, the graph shows resistance rather than resistivity becausecalculations of the latter are complicated by the exact details of theantenna design. Since we are only interested in maximising this value,and in achieving a suitable resistance for a particular sample, it issufficient to assess samples only in terms of the measured resistance,as is done here. In order to standardise these measurements, allresistances quoted in this description refer to a blunted bow-tieantenna (of the type shown in FIG. 1) with a 5 μm gap (a micrograph ofthe central region of this type of antenna is shown in the inset to FIG.5). This graph therefore shows that in the temperature range of 225° C.to 375° C., the resistance has increased to around 10 MΩ. In thistemperature range it is also apparent that there is only a slightincrease in lifetime from about 100 fs to 200 fs. These resistances areusable, particularly in the case of their use in receivers, and thelifetimes are excellent for use in receivers.

Between 375° C. and 475° C. a sharp increase in resistance occurs, by anorder of magnitude, which is a desirable result. The lifetime in thistemperature range also increases to between approximately 200 and 300fs. However, while the lifetime values have increased, they are stillquite usable, particularly for emitters, where although short lifetimesare required, the shortness is not as critical as it is for receivers.Further, for emitters, it is an advantage to have very high resistancesto maximise the electric field that can be applied to an antenna whilelimiting the current flow. Therefore annealing temperatures in the rangeof approximately 400 to 475° C. for LT-GaAs to be used in emitters ismost preferable.

Above 500° C. the lifetime deteriorates substantially, making thosetemperatures undesirable for LT-GaAs for uses where a low lifetime isrequired.

It is to be noted at 400° C. the data point shows a spurious increase inlifetime. It is considered that this is probably the result of astatistical fluctuation and that overall the graph establishes thegeneral trends as discussed. Overall, from this graph it can beconcluded that optimum annealing temperatures for LT-GaAs to be used inemitters is around 425° C. and around 325° C. for receivers.

FIG. 6 is a plot of the THz power received against frequency for tworeceivers formed from photoconducting antennas of the type describedwith reference to FIG. 1.

The upper trace corresponds to a receiver fabricated from wafer A. Thephotoconducting material was fabricated using the annealing temperatureof 325° C. for a period of 10 minutes.

The lower trace shows results from another receiver created using knowntechniques. The graph compares the THz power received by each receiveras a function of frequency.

This graph shows that the LT-GaAs receiver formed according to anembodiment of the present invention outperforms the other receiver,particularly in terms of sensitivity, and also with a slight improvementin bandwidth. Directly comparing the LT-GaAs formed in accordance withan embodiment of the present invention with the As—GaAs receiver, whichis one currently commonly used, it is worth noting that the improvementin sensitivity is close to an order of magnitude.

In addition to optimising the annealing temperature, it is also possibleto optimise the annealing duration. FIG. 7 provides a graph of peaksplitting shift obtained through x-ray diffraction measurements againstanneal time. To obtain this data, a wafer of LT GaAs grown at nominally200° C., on a straight GaAs substrate (wafer B) was examined. Anincrease in the change in x-ray splitting corresponds to a reduction inthe excess arsenic concentration.

FIG. 7 therefore demonstrates that saturation of the annealing effect isachieved after 15 minutes for the 340° C., 390° C. and 440° C.temperatures, although slightly longer for the 290° C. anneal, being thelower curve, although a sizeable proportion of the shift does occur inthe first fifteen minutes for that temperature.

From this graph it can be concluded that for annealing temperatures of340° C. and above, an annealing time of between ten and fifteen minutesis sufficient to provide the larger part of the change in the excessarsenic.

This conclusion is further supported by referring to FIG. 8, whichillustrates a plot of excess arsenic concentration versus anneal timefor wafer A. The data was obtained LT-GaAs wafer grown on AlAs using MBEat a temperature of 350° C. This graph shows that the arsenicconcentration decreases dramatically in the first ten minutes ofannealing, and only marginally thereafter.

The principles behind optimising the annealing process may be applied toany type of As implanted materials, particularly Group III-Vsemiconductors utilising arsenic, such as InGaAs, LT-AlGaAs, As—GaAs anddoped GaAs.

Considering LT-InGaAs, this is not a material conventionally utilised inradiation emission and detection systems, principally because it isconsidered difficult to increase its resistivity while maintaining a lowlifetime. This is because, the high annealing temperatures utilised increating InGaAs typically cause breakdown of the crystal quality.

InGaAs is typically created by first growing the material in an MBEchamber at 450° C. The choice of this conventional temperature isrelated to the fact at much higher temperatures the In species desorbsfrom the surface during growth, and hence is not incorporated into thecrystal. Annealing of LT-InGaAs conventionally occurs at 600° C., whichunfortunately provides enough energy to the InGaAs crystal to movearound the elemental species themselves, rather than just the excess As.

However, LT-InGaAs may be instead annealed at temperatures at or below500° C. An advantage of this is that it becomes possible topreferentially adjust the excess As, while leaving the InGaAs crystaluntouched.

FIG. 9 illustrates the properties of LT-InGaAs formed in accordance withan embodiment of the present invention. The graph shows the relationshipbetween lifetime and anneal temperature as well as resistance and annealtemperature.

In this case the wafer consisted of 1 micron of LT-In_(0.3)Ga_(0.7)Asgrown at nominally 230° C., grown on 500 nm AlAs grown at around 600°C., on a buffered GaAs substrate (wafer C). The exact alloy compositionis also nominal because it is not well known how preferentially the Inspecies will incorporate in low temperature MBE growth.

FIG. 9 shows a scatter graph of the variation in lifetime and resistanceas a function of the anneal temperature. It can be seen that for sometemperatures there is a variation in the measured lifetime, asmeasurements are performed on several nominally identical samples, andon different regions of the same sample. This is considered to be afacet of the difficulty of growing InGaAs on GaAs. It is to beappreciated that InGaAs is highly strained when grown on GaAs substratesdue to a mismatch of the lattice constants. This strain can relax withthe formation of defects and dislocations in the crystal structure, andhence in principle forming regions having microscopically differentproperties.

The inventors believe that the scatter is a result of these smallvariations. Nevertheless, the trend in material properties isestablished well by the scatter graph. It can be seen that the lifetimeimproves initially with anneal temperatures between 200° C. and 400° C.At 200° C. the lifetime is of the order of 2.2 ps and at 400° C. itdrops to around 0.6 ps. This decrease in lifetime is completely contraryto conventional wisdom and it is considered to occur because theannealing schedule at low temperatures initially reduces any crystallinedefects that may be present within the InGaAs layer, again due to itsdifficult growth. There is a clear minimum in the lifetimes in theregion of 400° C. anneal temperature, with the lowest recorded lifetimebeing 532 fs. Above 450° C. the lifetime is seen to rise.

The resistance at temperatures of 200° C. to 400° C. increases fromabout zero to 5 MΩ, which is a usable resistance. Above 450° C. theresistance drops.

The inventors conclude that the annealing temperature influencesLT-InGaAs in the following manner. At temperatures from 200 to 400° C.,the process of precipitation of excess As commences in similar fashionto that for GaAs, but this effect is outweighed by an improvement incrystalline quality that dramatically reduces the lifetime.

In the temperature range of 350-450° C. a clear increase in theresistivity due to the formation of As precipitates, and a minimum inthe lifetime, of the order of 500 to 600 fs is seen.

Above 450° C., the situation reverses with the resistance fallingsharply as the lifetime rises. When the material was annealed at evenhigher temperatures, the surfaces took on a different appearance,associated, as established by examination under an electron microscope,with a change in the surface morphology. Noting that these temperaturesmatch and then exceed the temperature for which InGaAs is grown in anMBE reactor, the inventors conclude that at these higher annealingtemperatures the InGaAs composition is itself affected on a microscopicscale.

Therefore, for InGaAs, the optimum annealing temperature range isapproximately 350° C. and 450° C. These temperatures produce LT InGaAswith a usable resistivity and a greatly improved lifetime. Such amaterial is capable of being used as a

photoconductive material in radiation emitters and receivers.

Variations and additions are possible within the general inventiveconcept as will be apparent to those skilled in the art. It will beappreciated that while exact annealing conditions will vary dependingupon the semiconductor material, the broad inventive concept of thepresent invention may be applied to any semiconductor material withsimilar characteristics, regardless of its chemical constituents, sothat the exact embodiment shown is intended to be merely illustrativeand not limitative.

FIG. 10 schematically illustrates an investigative system in accordancewith a further embodiment of the present invention.

In the investigative system of FIG. 10, InGaAs is used as thephotoconductive material for both the emitter and the detector. Theinvestigative system may be used for imaging or determining compositioninformation from structures.

In the investigative system laser 201 produces a 1 μm pulsed beam ofradiation 203 which is then divided by beam splitter 205 to form a probebeam 207 and a reference beam 209. As an example, the laser may be anNd:YAG or ND:YLF laser.

The probe beam 207 irradiates emitter 211 to produce a beam of THzradiation 213. The emitter 211 is a photoconducting antenna of thegeneral type described with reference to FIG. 1. The photoconductingmaterial is InGaAs which has been annealed at low temperatures inaccordance with the findings described with reference to FIG. 9.

The beam of THz radiation 213 is then directed via a series of mirrors215 onto sample 217, which is provided on a stage allowing the sample tobe scanned in two orthogonal directions. THz radiation, which isreflected from the sample, is then directed via a series of mirrors 219onto detected 221.

The detector 221 is also a photoconductive antenna, the photoconductivematerial being InGaAs. In order for the detector to detect THz radiationreflected from the sample, the detector is irradiated with pulses of 1μm radiation taken from the reference beam 209. In order to vary thephase of the reference beam 209 which reaches the detector 221, thereference beam is directed into a scanning delay line 223 which servesto continually increase and decreases the path of the reference beam andhence varies the phase of the reference beam as it reaches the detector221. The scanning delay line 221 operates under computer control 225.

In the above described non-limiting embodiment, both the emitter and thedetector are fabricated from LT-In_(x)Ga_(1-x)As. Thus, both the emitterand the detector have the advantage that they can be excited using acommonly available 1 μm laser. In this case the composition x, matchedto the exact laser wavelengths in the 1040 to 1070 region, ranges from0.2 to 0.3.

Beam splitter 205 is used to split the laser pulses so that both emitterand detector are photoexcited from separate optical systems. Thus, afurther variation on the above embodiment of a THz pulsed imaging (TPI)system, may include the use of frequency doubling optics, providedeither before beam splitter 205 or within the path of the reference beam207 or probe beam 209. These optics double the frequency (and energy) ofthe laser pulses, so enabling them to photoexcite higher bandgapsemiconductors, such as GaAs. Thus, in addition to a THz systemincorporating a 1 μm laser and LT-InGaAs emitters and receivers, ahybrid system may be envisaged with other combinations of InGaAsemitters and/or receivers, with frequency doubled laser pulses impingingon emitters and/or receivers formed from a different photoconductivematerial.

1. A method of determining optimal annealing conditions for asemiconductor material comprising: obtaining a first set of valuesindicative of resistivity of the material for a plurality of annealingtemperatures; obtaining a second set of values indicative of carrierlifetime of the material for a plurality of annealing temperatures; andcomparing the first and second set of values to determine an annealingtemperature or a range of annealing temperatures where the carrierlifetime and the resistivity of the material are optimized.
 2. Themethod of claim 1, further comprising: determining an optimum annealingduration for the material.
 3. The method of claim 2, wherein thematerial contains As, and the optimum annealing duration is determinedby obtaining a third set of values indicative of arsenic concentrationof the material for a plurality of annealing durations and for at leastone annealing temperature; comparing the at least one third set ofvalues with the first and second sets of values to determine anannealing duration and an annealing temperature which together optimizethe carrier lifetime and the resistivity of the material.
 4. A method ofenhancing characteristic properties of a semiconductor, the methodcomprising annealing a base material at a temperature of 475° C. or lessto form the semiconductor, the temperature being determined according tothe method of claim
 1. 5. The method of claim 4, wherein thecharacteristic properties enhanced includes carrier lifetime andresistivity.
 6. A method of producing a semiconductor material withphotoconductive properties, the method comprising annealing the basematerial at a temperature of 475° C. or less so as to enhance thecarrier lifetime of the material and the restivity of the material foruse as a photoconductor, the temperature being determined according tothe method of claim
 1. 7. The method of claim 4, wherein the annealingoccurs at a temperature in the range of 250° C. and 450° C.
 8. Themethod of claim 4, wherein the base material is grown using molecularbeam epitaxy.
 9. The method according to claim 4, wherein the basematerial is produced using As ion implantation.
 10. The method accordingto claim 4, wherein the base material is formed in a growth chamber andannealing occurs outside the growth chamber.
 11. The method according toclaim 4, wherein the semiconductor is a Group III-V semiconductor withphotoconductive properties.
 12. The method according to claim 4, whereinthe semiconductor comprises As.
 13. The method according to claim 4,wherein the base material is GaAs.
 14. The method according to claim 13,wherein the wherein the GaAs is grown in a molecular beam epitaxyreactor at a temperature in the range of approximately 200° C. to 300°C.
 15. The method according to claim 4, wherein the base material isInGaAs.
 16. The method of claim 15, wherein the base material isannealed at a temperature in the range of 350° C. to 450° C.
 17. Themethod according to claim 4, wherein the annealing is performed forfifteen minutes or less.
 18. A semiconductor material formed using themethod of claim
 1. 19. A photoconductive element comprising InGaAs, saidInGaAs having a carrier lifetime of at most 1 ps.
 20. A photoconductiveemitter comprising the semiconductor material of claim
 18. 21. Theemitter of claim 16, wherein the emitter is configured to emit terahertzradiation formed using a method according to claim
 1. 22. Aphotoconductive receiver comprising the semiconductor material of claim18.
 23. The receiver of claim 22, wherein the receiver is configured toreceive terahertz radiation.
 24. A photoconductive antenna comprising aphotoconducting substrate and two electrodes provided on the surface ofsaid photoconducting substrate, said photoconducting substratecomprising InGaAs having a carrier lifetime of less than 1 ps.
 25. Aninvestigative system comprising: a laser configured to emit a pump beamhaving a wavelength in the range from 1.3 and 1.55 μm, an emitterconfigured to emit emitted radiation in response to irradiation by saidpump beam; and a detector for detecting said emitted radiation, whereineither or both of the emitter or detector comprise InGaAs. 26-29.(canceled)